Continuous ink jet printing provides the fastest means of generating ink drops from the orifices of an individual printhead nozzle plate, and therefore it is valued for its ability to print at the highest feasible speeds with high image quality. The ink channel of a printhead nozzle plate supplying the orifices with ink is maintained under high fluid pressure to constantly propel an ink stream through the orifices, and a stimulation means is applied to create one or more uniform populations of drops from the ink stream that was ejected. The time lag for unpressurized refill of the ink channel of a drop-on-demand ink jet printhead is mitigated, and the drop firing frequency and printing speed can be increased. Continuous inkjet printing requires a means of intercepting constantly generated drops that are not intended to reach the printed substrate's surface, however. In prior commercial applications, electrostatic charging of ejected monomodal, uniformly sized drops was employed to enable dropwise electrostatic charge deflection by polarized electrodes, sending nonprinting drops onto a gutter surface and into a catcher return line that was under vacuum, e.g., as described in U.S. Pat. No. 4,636,808 to Herron and U.S. Pat. No. 5,801,734 to Schneider. The return line recycled the ejected but unprinted ink back to a central ink supply reservoir in a fluid management apparatus to be pumped back to the printhead under pressure for reuse. More recently, deflection strategies have emerged that use air impingement to manage the placement of bi- or multimodal drop populations, which are described, e.g., in U.S. Pat. Nos. 6,588,888 and 6,863,385 to Jeanmaire et al. Smaller nonprinting drops can be swept onto a gutter, then into an ink catcher, and finally be recycled through a return line, while the higher mass of a larger printing drop largely maintains its original trajectory. It then impacts the substrate to be printed, creating a digitally controlled impression mark.
The material compatibility properties of the electrostatic deflection process previously limited the scope of continuous ink jet ink compositions. Simple aqueous solutions of ionic dye colorants with few other functional ingredients worked the best. The prevalence of pigmented ink systems with polymer binders in consumer and office ink jet printing systems has highlighted the need for improved print permanence, durability and waterfastness in continuous ink jet applications. But the presence of ionic polymers in ink as binders or pigmented colorants with dispersants can lead to runtime problems due to material build-up from aerosol depositions, poor redissolution or redispersability of fouling ink deposits, the electrical shorting of charge leads, or even the physical instability of highly charged ink drops containing colloidal forms of material. The development of MEMS silicon-based nozzle plate devices to stimulate high frequency fluid stream break-up into bimodal drop populations, as disclosed in U.S. Pat. No. 6,682,182 to Jeanmaire et al., complements the air impingement strategies to deflect non-printing drops, and it has widened the latitude of compatible materials for aqueous continuous ink jet drop formation. Unfortunately, other continuous inkjet printer subsystems, which were optimized for inks based on simple aqueous ionic dye solutions, encounter difficulties with these more complex fluids based on pigmented colorants: in particular, the fluid delivery system.
The continuous inkjet printer fluid delivery system contains a main ink supply reservoir that feeds ink to a high pressure, low pulsation pumping system directing it to the printhead, and which receives any unprinted ink returned under vacuum from the printhead deflection, gutter and catcher systems. The jetted-and-returned ink is subject to evaporation of the carrier vehicle, and consequently the fluid delivery system contains a supply reservoir of a replenisher fluid to restore the solvent level of the ink. If the ink became too concentrated, the delicate drop control processes could be affected by viscosity changes, and printed image qualities could also change, producing visual density and hue deviations. When the ink reservoir fluid level is lowered by ink use in printing or by solvent evaporation due to extended ink recycling in a print-ready idle state, there is an opportunity to adjust the used ink concentration level by supplying the ink reservoir with either an ink vehicle replenisher fluid or with more fresh ink. In order to determine the state of concentration of the recycled ink, a variety of in-line analytical methods have been tried, but the quantification of the fluid ionic conductivity (or its reciprocal, the fluid resistivity) of the ink supply aqueous ink has proven most effective. Dye-based aqueous inks contain mobile, ionic organic colorants, and a calibrated in-line cell can measure the fluid's ability to support charge transport, which is directly proportional to the concentration of the colorant. A concentrated dye ink will show increased conductivity (and reduced resistivity), and the fluid delivery system controller can be programmed to correctly restore the ink reservoir fluid level with a solvent replenisher fluid in that circumstance, as described, e.g., in U.S. Pat. Nos. 3,761,953, 5,526,026, and EP 0 597 628. Pigmented inkjet inks typically may also contain ionic organic materials, but their physicochemical properties are profoundly different. Typically, the ionic material is derived from a base neutralized polymeric dispersant or a polymeric binder. The fluid ionic strength may be reduced, and the ionic mobility of the chemical species responsible for charge transport is much lower. The net result is that the fluid ionic conductivity is substantially reduced, and the fluid resistivity is much higher.
A second recirculation problem occurs with pigmented inks. After jetting, the deflected, guttered and caught ink is emulsified with air in the return process, and the ink can take up carbon dioxide. The carbon dioxide forms carbonic acid, which lowers the pH of the alkaline ink by protonating the available bases. In doing so, additional ionic species can be created, reducing the used ink resistivity even thought the colorant strength may not have changed. Thus the replenishment cycle may be confounded, resulting in poorer control of the recycled ink concentration. The reduced ionic conductivity of even a concentrated continuous ink jet pigment ink exacerbates the risk of adventitious disturbance of its colorant-resistivity ratio by carbonic acid or by trace amounts of ionized components in a replenisher fluid. Furthermore, the pH drop resulting from carbon dioxide uptake is a potential concern for the stability of the dispersed pigment colorant. The broad class of styrene-acrylic copolymers is very useful in inkjet inks, and the organic polymer is solubilized by the ionization of the carboxylic acid functional group of the acrylic acid moiety. Due to effects of the polymer conformation and the solvation state of the carboxylate group, its basicity may be considerably higher than simple model aliphatic carboxylate salts in water might suggest. A modest reduction in ink pH could induce colloidal instability and aggregation of the pigment particles, leading to the blocking of in-line fluid filters that protect the printhead nozzles from oversize particles that would plug them. Similar considerations apply for self-dispersed pigments that are oxidized to create solubilizing, surface-bound carboxylate salts. An organic pH buffer to stabilize drop-on-demand ink jet inks comprised of surface-treated pigment for consistent properties during storage, which is selected from the Good's buffers and employs an aminopropanediol derivative, is disclosed in U.S. Pat. No. 7,370,952 to Inoue et al; no mention of conductivity requirements for ink in continuous ink jet ink recycling is made. U.S. Pat. No. 5,830,264 to Fujioka et al. discloses an ink containing a dye colorant and a 2-amino-1,3-propanediol derivative as an organic weak base, where the pH that resulted ranged from 8 to 10; ink recycling is not disclosed, and the addition of a relatively strong protic acid to prepare the conjugate acid of the organic weak amine base is not taught.
Therefore the need exists to modify the chemical composition of pigmented continuous ink jet inks in a manner compatible with their unique material properties, in order to facilitate accurate fluid system replenishment and simultaneously protect the colloidally dispersed pigment against potentially destabilizing pH drift.